Antennas comprised of multiple radiating elements arranged as arrays are used in many applications where it is necessary to electronically steer the antenna's radiating pattern (beam). They are also used in applications that derive the direction from which a signal arrives by measuring the relative amplitudes and phase angles between each of the antenna elements for a given received signal. For each of these types of applications it is necessary to know about any system induced phase and amplitude offsets and errors between each element so that they can be removed from received signal measurements and compensated for in transmit signal generation. The process of determining the system induced phase and amplitude offsets and errors is referred to as calibration of the system.
Calibration can be accomplished as a one-time measurement in a lab or at an installation site or it can be a periodic process to eliminate variation over time due to the environmental effects of temperature, altitude, humidity, etc. When periodic calibration is required, it is generally provided for as a built-in function of the system itself and requires no external support. This process is called self calibration. Self calibration is common in airborne applications where extreme environmental variations exist as well as highly limited access to the systems during use.
The Traffic Collision Avoidance System (TCAS) is one such system that utilizes a multi-element antenna to steer the beam during transmit and to determine angle of signal arrival during receive. Typical TCAS systems use a 4-element antenna with each element having a dedicated signal feed point. The following is a brief description of how a 4-feed antenna system is calibrated. Since each element is mutually coupled to each other element and since the electrical distance between the elements is known based on the physical spacing of the elements, the phase and amplitude offsets and/or errors for each element can be determined by measuring the complex transfer function (i.e. the insertion phase and amplitude) between different pairs of elements and using the results of these measurements to calculate all of the relative offsets and/or errors in the system. This technique will work with any number of elements greater than or equal to three since it relies on differential measurements of phase and amplitude between one element and any two other elements.
New systems are being developed that will use antennas with only two elements, thus eliminating the possibility of using the calibration method described above.
For example, commonly owned U.S. Pat. No. 7,583,223, which is hereby incorporated by reference as if fully set forth herein, discloses a system that includes a first antenna and a second antenna located on a top surface of an aircraft, spaced apart along a first axis, as well as a third antenna and a fourth antenna located on a bottom surface of the aircraft, spaced apart along a second axis orthogonal to the first axis. The system also includes a transmitting, receiving, and processing system coupled to the first, second, third, and fourth antennas, wherein the transmitting, receiving, and processing system is configured to transmit TCAS interrogations, receive TCAS replies, and process the TCAS replies to determine the relative bearing of a second aircraft from the first aircraft. Such a system provides a TCAS antenna system, employing two pairs of two-element arrays, that uses less cabling than previous attempts, specifically, four cables (two to top and two to bottom) instead of eight cables (four to top and four to bottom).
Commonly owned U.S. Pat. No. 4,855,748 discloses an approach by which cables associated with four-element antenna arrays may be phase calibrated. However, as above alluded to, such approach cannot be employed for phase calibration of the above-described two-element arrays.